Method of making selenium elements



Dec. 12, 1944. A, J. MILLER ET AL METHOD OF MAKING SELENIUM ELEMENTS Filed May 1, 1943 ATTORNEY Patented Dec. 12, 1944 METHOD OFMAKING SELENIUM ELEMENTS Arthur, J. Miller, Newark, N. J and Irving Richard Taylor, Brooklyn, N. Y., assignors to Federal Telephone and Radio Corporation, New York, 1 acorporation of Delaware I Application May 1, 1943, Serial No. 485,384

4 Claims. (Cl. 175-366) This invention relat'e'sto selenium elements and has for its obaect'to improve the adherence of the selenium to its base plate and to improve the electrical characteristics of the elements.

It is known that selenium elements such as rectlfiers can be made by sprinkling selenium powder on a base plate and compressing the powder against the plate, preferably at an elevated temperature. This causes the powder particles to compact and adhere to each other and to the base plate. Following this treatment the selenium-coated element may then be heat treated at a high temperature somewhat below the melting point of the selenium to complete the crystallization of the selenium in a well-known manner; after which the usual counterelectrode may be applied to the selenium surface and the element electro-formed in a usual manner.

We have found that better adherence of the selenium layer to the base plate may be obtained by heating the selenium to a higher temperature than is desired for the best electrical properties of the element. The temperature during the compression which gives the best electrical rectitying properties is around 80 to 150 C, while the temperature giving best adherence to the base plate during the compression is higher and may well be near or even higher than the melting point of selenium which is around 217 C. In accordance with our invention we apply temperatures during the compression treatment which are different at the surface of the selenium layer than at the base plate, the base plate temperature being higher than the surface temperature. In this way a good adherence at the base plate may be obtained while providing a suitable treatment at the surface to give good electrical and rectifying properties.

The invention will be better understood from the following detailed description taken in conjunction with the drawing which comprises a perspective view of a disc with aligned compressing and heating members. The drawing shows a base plate or disc I which is shown with a centrally located hole 2 such as is often used in practice. The plate is covered with a layer of selenium powder 3 which may be provided by grinding selenium which has been melted and then solidifled. The base plate may be of iron, steel. aluminum, or the like and may be roughened and nickel coated according to a common practice. The

selenium covered plate is shown in position between the platens 4 and 5 of a press so that when the platens are brought together. the selenium powder layer 3 is compressed against the base plate I. The pressure is not critical although it should be sui'flcient for a good adherence of the selenium to the carrier plate. A pressure of around 4000 pounds per square inch has been found suitable.

Provision is made for heating the press platens, these being shown as the electrical heating element 6 associated with the lower platen 4 and electrical heating element 1 associated with the upper platen 5. The lower platen is heated hotter than the upper platen. A suitable temperature range for the lower platen may be, for example, from 110 C. to a temperature around the melting point of selenium which is around 217 C. and a suitable temperatur range fOr the upper platen may be from 20 C. to 110 C. for example. The application of the elevated temperature during the compression to the selenium layer serves to soften the selenium causing the particles to adhere to each other as they are packed together, forming a solid layer of selenium adhering to the carrier plate. The time required for the compression need not be long, for example, one or two minutes or perhaps a little longer.

After this compression treatment the element may then be further heat treated without pressure according to a well-known process, for example by placing the selenium element in an oven at a temperature somewhat below the melting point of selenium, for example, around 200 C. for around a half hour or even longer. This produces a complete crystallization of selenium rendering it conductive and therefore useful as an electrical element. The selenium surface may then be treated, if desired, by any of a number of well-known treatments for improving the blocking layer and the counterelectrode may then be applied. The counterelectrode may, for example, be an alloy such as Woods metal which'may be melted and sprayed on. When cooled, it forms a closely adherent contact layer in intimate contact with the selenium surface. It is then the common practice to electro-form the element by passing a D. C. or a pulsating D. C. current in the direction opposite from that of normal current flow. This builds up the barrier layer and improves the rectifying and voltage resisting characteristic. 7

By our step of heating the base plate to a higher temperature than the exposed surface of selenium, a good adherence of the selenium layer is had to its base plate so that the selenium cannot readily be knocked off. At the same time the best temperature for a good electrical quality of the blocking layer is had at the surface or the selenium, thereby resulting in th most eflective rectification ratio and voltage resisting properties. If it were attempted to heat the entire element at the temperature which is'best-for good adherence the electrical and rectifying properties would not be as good, and likewise if it wereaattempted to heat the entire element at the best temperature for good electrical and rectifying properties, the adherence would not be so'good. But by the use of the difierenttemperatures above and below the element the best temperatures for both purposes are had at the same time.

What is claimed is:

1. The method of making a selenium element which comprises compressing selenium powder against a base plate and maintaining the carrier plate temperature higher than the temperature of the selenium surfaceduring the compression.

2. The method of making a. selenium element which comprises compressing a-base plate having a layer of selenium powder between the platens Lora press. 'the s-preu platen adjacent the plate being at a'rnizherrtemperature :than the platen taining a. layer :of selenium ypowder between the platens -01 a press with "the platen adjacent the :powderratatempemtureinatherange-oi.about 20 C. 'to 110 -.'C., the temperature of the platen adjacent the-'plate'l being'in therang of about 110* C. to 217 C.

:4. The method aselenium element which (comprises 'compressing'a :base plate and a layer-of selenium gpowder thereon between the platens of a. gpl'essxatha. pressure in the general order rof -'4;000 pounds Per square inch with the platen tadjacenirthe base .plate at a temperature in *thesrange 0! about I110 C. to 217 C. and the platen adiarzent the l-seleniumgpowder in the range 0f.20 C.'t0. 110 C.

J. MILLER. 'R. TAYLOR. 

